The present invention relates to rechargeable battery cells, typically of a flat, multi-layer, laminate form, which are folded, accordion style, to achieve a physically compact, yet high electrode area, battery structure having a resultant high electrical energy storage and power capacity. In particular, the invention provides such a folded battery cell structure which is less susceptible to laminate element damage, especially in the stressed outer fold regions, and also describes a method of fabricating such a cell structure which is capable of being readily, economically, and precisely practiced in commercial application.
Battery cell structures of general concern comprise an assembly of flat, planar, sheet-like elements which include positive and negative electrodes and an interposed separator, along with electrically conductive current collector elements associated with the respective electrodes. In a widely utilized structure of this type, such as described, for example, in U.S. Pat. Nos. 5,460,904 and 5,554,459, the disclosures of which are incorporated herein by reference, the compositions of the electrode and separator elements comprise polymeric matrix components which enable the fabrication of a thin, flat unified cell structure by means of simple heat and pressure lamination. The multi-fold cell structure is usually effected by repeated reverse, or zigzag, folding of an elongate cell of selected lateral dimension at spaced parallel axes to achieve the desired longitudinal dimension of the cell.
While the basic laminated structure of a cell of this type is highly advantageous in its maintaining the essential close physical contact between the laminar elements, the accompanying restriction of relative lateral movement between such elements during a folding operation tends to result in stresses within the elements, particularly in the region of the extended outer circumferences of the folds. Such stresses can lead to breaks and compression creases in the electrodes and the incorporated current collector elements, either of which may result in disruption of cell operation. The normally thin profile of a single cell structure, i.e., a cell comprising a positive electrode, a negative electrode and one intervening separator, and the flexibility of the polymeric component generally keep these stresses to an acceptable level. However, the use of increasing ratios of solid active electrode materials reduces the overall flexibility of the laminate cell sheets and results in an increase of folding stress.
This deleterious effect is greatly aggravated in bicell structures which are designed to increase energy storage capacity by including an additional electrode, current collector and separator element in the laminate assembly and often results in cell component damage and interrupted operation. With a pair of like polarity electrodes and associated separator and current collector elements laminated to the opposite surfaces of a common intermediate opposite polarity electrode element, the complete bicell has an increased composite thickness which increases the likelihood of damaging stress in the now vastly extended outer circumference region of a cell structure fold.
Various attempts have been made to alleviate this problem of excessive stress on cell structure components arising from the inordinate degree of outer laminate cell element extension in the regions of transverse folding. The geometry of the folded cell structure, however, severely limits the available solutions to those which in effect eliminate structural material from cell fold regions. However, this solution results in a cell structure in which exterior laminate elements, namely, the electrodes and associated current collectors, are reduced from continuous sheets to a series of separate elements, thus leading to a mere stack of electrodes superficially associated through an intermediate separator medium. Such a single cell structure is described in U.S. Pat. No. 5,498,489. This structure suffers from the major disadvantage of a lack of means to ensure the alignment of the overlying opposed electrodes in precise register in order to achieve optimum cell capacity, such capacity being a function of the area encompassed by the overlap of opposing electrode surfaces.
Other stacked electrode cell structures of single cell type are shown in U.S. Pat. Nos. 5,525,441, and 5,667,909, which structures similarly lack means for ensuring efficient electrode alignment and further suffer from the need for a multiplicity of conductor members to connect each of the multiple electrodes in order to utilize the stored energy. This adds to the intricacies of cell fabrication, and also results in accumulated electrical resistance which seriously detracts from the desired increase in cell energy.
The present invention provides a multi-fold laminated cell rechargeable battery structure which relieves to a remarkable extent the stresses normally developed in the region of cell element folds and thereby prevents cell element damage even in preferred, higher capacity bicell structures. In particular, in accordance with the present invention, prior to the final cell lamination, portions of each of the outer electrode members are removed from the immediate lateral regions of the intended cell folds. The portions removed result in the formation of slots adjacent to regions of intact electrode material in the electrode members. The battery cell is then assembled in such a manner that the slots in the first electrode member are aligned with the regions of intact electrode material in the second electrode member and the slots in the second electrode member are aligned with the regions of intact electrode material in the first electrode member. In the case of a bicell structure the present invention ensures that the maximum thickness in the fold regions will not be significantly greater than the thickness of a single cell type structure. This limited structure thickness can tolerate the subsequent folding operation without significant laminate element damage.
The slots formed in the outer electrode elements effectively prevent fold stresses, while the adjacent regions of intact electrode material are sufficient to maintain prelamination longitudinal integrity and resistance to shifting displacement or distortion. This helps in the ready, precise alignment of cell elements without resort to extraordinary assembly means. In particular, simple alignment of the peripheries of the individual sheet elements of a cell according to the present method will ensure proper registry of the slotted regions and of the overlapping electrode segments. This is unlike prior art practices in which separate registry of individual electrode elements was required and often hindered by intervening opaque cell elements. Lamination of the aligned elements fixes the cell elements in proper position for subsequent folding of the assembly to achieve the desired multi-fold cell. In addition, the continuity of the associated current collector elements is maintained along the entire extended cell structure which allows the use of a minimum of conductor to provide connection terminals.
The present invention may be employed with any combination of the electrode, separator, collector, and electrolyte elements and compositions in current use in the industry, particularly those of the type described in the aforementioned incorporated patents. In addition to the cast polymeric composition separator elements, pre-formed sheets of microporous polymer, such as commercially available fabrications of polyolefins, may be used.